1. Field of the Invention
This invention relates to a silicon oxide film forming technique which is intended to improve yield, quality, uniformity during formation of a silicon oxide film on a silicon surface.
2. Related Background Art
Silicon oxide film forming methods may include the thermal oxidation method, the vacuum deposition method, the CVD method, etc., and generally the thermal oxidation method has been utilized. The thermal oxidation method comprises heating a silicon substrate in an oxidative atmosphere to a predetermined high temperature, thereby forming a film of silicon oxide on the surface of the substrate.
The device for forming a silicon oxide film by the thermal oxidation method of the prior art is constituted of a heating portion 3, a uniform heating tube portion 2 and a quartz tube portion 1, as shown in FIG. 1, and has a structure capable of introducing various gases into the quartz tube portion 1. In the device of the prior art shown in FIG. 1, a plural number of gas introducing pipes are connected to the quartz tube portion. That is, an oxygen gas introducing hole is positioned at the end of the quartz tube and an hydrogen gas introducing hole is positioned in a high temperature region in the quartz tube away from the oxygen gas introducing hole so that the hydrogen gas supplied may be efficiently subjected to combustion in a short time and that stable combustion with little change in the spread of the flame may be obtained.
In the prior art, oxygen gas is flowed through the introducing pipe 6 to fill internally the quartz tube portion 1 with oxygen atmosphere and, 2 to 5 minutes later, hydrogen gas is flowed through the introducing pipe 7 for combustion. At that time, the surface of the substrate made of silicon 9 located within the quartz tube is oxidized with the flow of the water vapor generated. In this case, it is generally practiced to control the oxide film thickness by temperature and time.
However, in the prior art device as described above, the mixing of hydrogen gas and oxygen gas cannot be sufficient, and the water vapor stream also tends to be nonuniform. Therefore, the device had the problems such as nonuniformity of silicon oxide film thickness within the plane of a wafer, and difference in silicon oxide film thickness between the front portion and the rear portion within the quartz boat, and great variance relative to a desired value, etc. The present inventor has discovered nonuniformity of the silicon oxide film thickness results from the fact that the hydrogen gas introducing hole is distant from the oxygen gas introducing hole.